Objective. Obsessive-compulsive disorder (OCD) is a chronic disorder of unknown etiology. An augmentation strategy is an approach for treatment-resistant OCD. This study was planned to assess the effect of atorvastatin on treatment-resistant OCD. Methods. This 12-week-long double-blind randomized trial was performed on 26 adult patients with treatment-resistant OCD. They were diagnosed with this kind of disorder based on the DSM-IV-TR. The patients were randomized to receive either 10 mg/day atorvastatin or placebo. The Yale-Brown scale was assessed at the baseline and 12 weeks later. Results. There were significant reductions in the obsession subtotal scoreof the Y-BOCS (p = 0.017) and the total Y-BOCS score (p = 0.041) in the atorvastatin group. Hence, the reduction in the Y-BOCS compulsive score (p = 0.081) was not statistically significant. Atorvastatin was generally well tolerated. There was a significant reduction in libido in the atorvastatin group (p = 0.019). Conclusions. The results of this study should be interpreted in the shadow of its restrictions. Some of the restrictions were a limited number of patients in the trial, a 12-week-long time Iranian Randomized Clinical Trial Registry Code: IRCT201511217290N2. trial, and not measuring NO before and after the study. It is recommended that researchers should consider these items in similar type of studies.